Using Sigfox and UART 1 causes instability

I am suspecting the default UART 1 pins are shared wit the Sigfox pins because when I hook up a NMEA GPS module over UART 1, anything I do with an object created from the Sigfox module may hang the SiPy.
The simplest way to reproduce is to run

sigfox = Sigfox(mode=Sigfox.SIGFOX, rcz=Sigfox.RCZ4)

soft reboot (CTRL-D)

sigfox = Sigfox(mode=Sigfox.SIGFOX, rcz=Sigfox.RCZ4)

hang 100%

There is no way to reset the sigfox without creating the sigfox object ?